312 research outputs found
General relativistic polarized radiative transfer: building a dynamics-observations interface
The rising amount of polarized observations of relativistic sources requires
the correct theory for proper model fitting. The equations for general
relativistic (GR) polarized radiative transfer are derived starting from the
Boltzmann equation and basic ideas of general relativity. The derivation is
aimed at providing a practical guide to reproducing the synchrotron part of
radio & sub-mm emission from low luminosity active galactic nuclei (LLAGNs), in
particular Sgr A*, and jets. The recipe for fast exact calculation of
cyclo-synchrotron emissivities, absorptivities, Faraday rotation and conversion
coefficients is given for isotropic particle distributions. The multitude of
physical effects influencing simulated spectrum is discussed. The application
of the prescribed technique is necessary to determine the black hole (BH) spin
in LLAGNs, constraining it with all observations of total flux, linear and
circular polarization fractions, and electric vector position angle as
functions of the observed frequency.Comment: 9 pages, 3 figures, accepted by MNRA
Tuning the second-harmonic generation in AlGaAs nanodimers via non-radiative state optimization [Invited]
Dielectric nanocavities are emerging as a versatile and powerful tool for the linear and nonlinear manipulation of light at the nanoscale. In this work, we exploit the effective coupling of electric and toroidal modes in AlGaAs nanodimers to locally enhance both electric and magnetic fields while minimizing the optical scattering, thereby optimizing their second-harmonic generation efficiency with respect to the case of a single isolated nanodisk. We also demonstrate that proper near-field coupling can provide further degrees of freedom to control the polarization state and the radiation diagram of the second-harmonic field
Sagittarius A* Accretion Flow and Black Hole Parameters from General Relativistic Dynamical and Polarized Radiative Modeling
We obtain estimates of Sgr A* accretion flow and black hole parameters by
fitting polarized sub-mm observations with spectra computed using
three-dimensional (3D) general relativistic (GR) magnetohydrodynamical (MHD)
(GRMHD) simulations. Observations are compiled from averages over many epochs
from reports in 29 papers for estimating the mean fluxes Fnu, linear
polarization (LP) fractions, circular polarization (CP) fractions, and electric
vector position angles (EVPAs). GRMHD simulations are computed with
dimensionless spins a_*=0,0.5,0.7,0.9,0.98 over a 20,000M time interval. We
perform fully self-consistent GR polarized radiative transfer using our new
code to explore the effects of spin a_*, inclination angle \theta, position
angle (PA), accretion rate Mdot, and electron temperature Te (Te is reported
for radius 6M). By fitting the mean sub-mm fluxes and LP/CP fractions, we
obtain estimates for these model parameters and determine the physical effects
that could produce polarization signatures. Our best bet model has a_*=0.5,
\theta=75deg, PA=115deg, Mdot=4.6*10^{-8}M_Sun/year, and Te=3.1*10^10K at 6M.
The sub-mm CP is mainly produced by Faraday conversion as modified by Faraday
rotation, and the emission region size at 230GHz is consistent with the VLBI
size of 37microas. Across all spins, model parameters are in the ranges
\theta=42deg-75deg, Mdot=(1.4-7.0)*10^{-8}M_Sun/year, and Te=(3-4)*10^10K.
Polarization is found both to help differentiate models and to introduce new
observational constraints on the effects of the magnetic field that might not
be fit by accretion models so-far considered.Comment: 19 pages, 11 figures, accepted to Ap
Study of the diffraction pattern of cloud particles and the respective responses of optical array probes
Optical array probes (OAPs) are classical instrumental
means to derive shape, size, and number concentration of cloud and
precipitation particles from 2-D images. However, recorded 2-D images are
subject to distortion based on the diffraction of light when particles are
imaged out of the object plane of the optical device. This phenomenon highly
affects retrievals of microphysical properties of cloud particles. Previous
studies of this effect mainly focused on spherical droplets. In this study we
propose a theoretical method to compute diffraction patterns of all kinds of
cloud particle shapes in order to simulate the response recorded by an OAP.
To check the validity of this method, a series of experimental measurements
have been performed with a 2D-S probe mounted on a test bench. Measurements
are performed using spinning glass discs with imprinted non-circular opaque
particle shapes.</p
Universal Superfield Action for Partial Breaking of Global Supersymmetry in D=1
We explicitly construct N=4 worldline supersymmetric minimal off-shell
actions for five options of 1/2 partial spontaneous breaking of
Poincar\'e supersymmetry. We demonstrate that the action for the N=4 Goldstone
supermultiplet with four fermions and four auxiliary components is a universal
one. The remaining actions for the Goldstone supermultiplets with physical
bosons are obtained from the universal one by off-shell duality
transformations.Comment: 9 pages, LaTeX file, PACS numbers: 11.30.Pb, 03.65.-
N=4 supersymmetric Eguchi-Hanson sigma model in d=1
We show that it is possible to construct a supersymmetric mechanics with four
supercharges possessing not conformally flat target space. A general idea of
constructing such models is presented. A particular case with Eguchi--Hanson
target space is investigated in details: we present the standard and quotient
approaches to get the Eguchi--Hanson model, demonstrate their equivalence, give
a full set of nonlinear constraints, study their properties and give an
explicit expression for the target space metric.Comment: LaTeX, 9 page
Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter VLBI Observations of Sagittarius A*
Millimeter very-long baseline interferometry (mm-VLBI) provides the novel
capacity to probe the emission region of a handful of supermassive black holes
on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole
at the center of the Milky Way, this provides access to the region in the
immediate vicinity of the horizon. Broderick et al. (2009) have already shown
that by leveraging spectral and polarization information as well as accretion
theory, it is possible to extract accretion-model parameters (including black
hole spin) from mm-VLBI experiments containing only a handful of telescopes.
Here we repeat this analysis with the most recent mm-VLBI data, considering a
class of aligned, radiatively inefficient accretion flow (RIAF) models. We find
that the combined data set rules out symmetric models for Sgr A*'s flux
distribution at the 3.9-sigma level, strongly favoring length-to-width ratios
of roughly 2.4:1. More importantly, we find that physically motivated accretion
flow models provide a significantly better fit to the mm-VLBI observations than
phenomenological models, at the 2.9-sigma level. This implies that not only is
mm-VLBI presently capable of distinguishing between potential physical models
for Sgr A*'s emission, but further that it is sensitive to the strong
gravitational lensing associated with the propagation of photons near the black
hole. Based upon this analysis we find that the most probable magnitude,
viewing angle, and position angle for the black hole spin are
a=0.0(+0.64+0.86), theta=68(+5+9)(-20-28) degrees, and xi=-52(+17+33)(-15-24)
east of north, where the errors quoted are the 1-sigma and 2-sigma
uncertainties.Comment: 15 pages, 10 figures, submitted to Ap
Distribution of sizes of erased loops of loop-erased random walks in two and three dimensions
We show that in the loop-erased random walk problem, the exponent
characterizing probability distribution of areas of erased loops is
superuniversal. In d-dimensions, the probability that the erased loop has an
area A varies as A^{-2} for large A, independent of d, for 2 <= d <= 4. We
estimate the exponents characterizing the distribution of perimeters and areas
of erased loops in d = 2 and 3 by large-scale Monte Carlo simulations. Our
estimate of the fractal dimension z in two-dimensions is consistent with the
known exact value 5/4. In three-dimensions, we get z = 1.6183 +- 0.0004. The
exponent for the distribution of durations of avalanche in the
three-dimensional abelian sandpile model is determined from this by using
scaling relations.Comment: 25 pages, 1 table, 8 figure
Exact and near backscattering measurements of the linear depolarisation ratio of various ice crystal habits generated in a laboratory cloud chamber
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/).Ice clouds were generated in the Manchester Ice Cloud Chamber (MICC), and the backscattering linear depolarisation ratio, δ, was measured for a variety of habits. To create an assortment of particle morphologies, the humidity in the chamber was varied throughout each experiment, resulting in a range of habits from the pristine to the complex. This technique was repeated at three temperatures: −7 °C, −15 °C and −30 °C, in order to produce both solid and hollow columns, plates, sectored plates and dendrites. A linearly polarised 532 nm continuous wave diode laser was directed through a section of the cloud using a non-polarising 50:50 beam splitter. Measurements of the scattered light were taken at 178°, 179° and 180°, using a Glan–Taylor prism to separate the co- and cross-polarised components. The intensities of these components were measured using two amplified photodetectors and the ratio of the cross- to co-polarised intensities was measured to find the linear depolarisation ratio. In general, it was found that Ray Tracing over-predicts the linear depolarisation ratio. However, by creating more accurate particle models which better represent the internal structure of ice particles, discrepancies between measured and modelled results (based on Ray Tracing) were reduced.Peer reviewe
Reversals in nature and the nature of reversals
The asymmetric shape of reversals of the Earth's magnetic field indicates a
possible connection with relaxation oscillations as they were early discussed
by van der Pol. A simple mean-field dynamo model with a spherically symmetric
coefficient is analysed with view on this similarity, and a comparison
of the time series and the phase space trajectories with those of paleomagnetic
measurements is carried out. For highly supercritical dynamos a very good
agreement with the data is achieved. Deviations of numerical reversal sequences
from Poisson statistics are analysed and compared with paleomagnetic data. The
role of the inner core is discussed in a spectral theoretical context and
arguments and numerical evidence is compiled that the growth of the inner core
might be important for the long term changes of the reversal rate and the
occurrence of superchrons.Comment: 24 pages, 12 figure
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